Noble metals such as platinum possess important catalytic properties for a range of chemical reactions. Such catalytic properties relate to the superior ability of these metals to absorb and dissociate hydrogen, carbon monoxide, sulfur, nitrogen oxides and various other molecules. For example, platinum has been used in automotive applications as an active component for catalyzing decomposition of various toxic exhaust gases. An important catalytic property of platinum is its ability to absorb and dissociate chemical species such as hydrogen and oxygen gases. This catalytic property has allowed platinum and its alloys to be used as catalysts for commercially important partial oxidation and reduction reactions in pharmaceutical compound manufacturing and in low-temperature fuel cells. Proton exchange membrane fuel cells (PEMFCs) using hydrogen as fuel have been important in the development of clean energy sources, and direct methanol fuel cells (DMFCs) have been developed as power sources for portable microelectronic devices. PEMFCs are also being developed as potential power sources for microeletronic devices such as notebook computers.
One challenge relating to the use of platinum catalysts is the high cost of platinum. Because of this high cost, it would be advantageous to be able to reduce consumption of platinum without sacrificing catalytic performance in practical applications. The high activity of noble metal catalysts may be closely related to the shape of the catalysts. Electrocatalysts used in PEMFCs and DMFCs have traditionally been made of carbon-black supported nanoparticles of platinum and platinum alloys, such as PtRh, PtCo, PtFe and PtNi. Commercially available electrocatalysts include, for example, porous carbon supported platinum nanoparticles sold under the name Vulcan XC-72R by E-TEK. Small particle size is believed to be important to achieve a catalyst having a large surface area.
Improving the sluggish kinetics of the electrocatalytic oxygen reduction reaction (ORR) may be critical to advancing hydrogen fuel cell technology. An important threshold value in ORR catalyst activity is a four-fold improvement in activity per unit mass of platinum (Pt) over the current commercial carbon-supported Pt catalyst (Pt/C) used in the vehicle fuel cells, which could allow fuel-cell powertrains to become cost-competitive with their internal-combustion counterparts.
Surface segregation is a metallographic behavior that may occur in bulk binary alloys based on the miscibility of bimetallic phase diagram in which chemical composition at surface differs from that in bulk. Based on both theoretical studies and some experimental data, when the dimension or crystal domain of a bimetallic alloy is reduced to the nanoscale, the miscibility between the component metal elements may be increased due to the larger fraction of atoms at surface or interfacial regions. Thus, an alloy with an even distribution of components may be produced. However, it may be possible to reconstruct the surface atoms of nanoparticles through post-synthesis treatments. Calculations indicate that segregation on surfaces of metal catalysts can influence the catalytic performance by modulating the binding energy and surface geometry of the metal surface and reactant molecules.